1. Field of the Invention
The present invention relates to control of irradiation energy to a sensitive object and control for achieving uniform illumination thereon, and more particularly to an exposure control device for exposure control and achievement of uniform illumination in an exposure apparatus utilizing a pulsed laser such as an excimer laser as the exposing light source.
2. Related Background Art
A conventional energy control device, for use, for example, as the exposure control device in an exposure apparatus for semiconductor device manufacture, is disclosed in the Japanese Laid-open Patent Sho No. 60-169136.
In said device, the fluctuation in the exposure energy given to a sensitive member, such as a wafer coated with photoresist, is reduced by dividing the exposure into a coarse exposure giving an exposure energy slightly smaller than the appropriate exposure, and a correcting exposure providing the necessary remaining exposure energy.
More specifically, in case of the exposure of one shot consisting of plural pulses, an optimum exposure is obtained by control with final pulses of smaller energy.
The above-mentioned shot means, in case of collective exposure method, the irradiation of an entire wafer with the exposure energy through a mask, or, in case of step-and-repeat method, the irradiation of a partial area of the wafer with the exposure energy.
However the laser beam employed as the energy source may result in unevenness in illumination intensity, called speckles, on the exposure plane, due to coherence of the laser beam, and such unevenness undesirably affects the line width control in the photolithographic process in the semiconductor device manufacture. A speckle reduction has therefore been proposed in the U.S. Pat. No. 4,619,508.
The speckle reduction in said patent consists of causing a two-dimensional vibration (raster scanning) of the laser beam with a predetermined period for example with a vibrating mirror, thereby slightly displacing the illuminating path of the laser beam and thus reducing the spatial coherency. If pulsed energy, such as an excimer laser beam, is applied to such illuminating system, plural pulses will be generated in synchronization with the two-dimensional scanning performed for example with a mirror. The pulse duration of an excimer laser is normally as short as about 20 nsec, so that each pulse of the excimer laser beam remains almost still in said two-dimensional vibration, when the mirror vibration is conducted with a frequency of several tens of cycles per second. FIG. 13 is a plan view of light source images (laser spots) on the pupil plane of the projection lens, obtained by the scanning motion of the laser beam according to the method disclosed in the above-mentioned U.S. Patent. It is assumed, in the exposure optical system, that the exit end of an optical integrator 28 is positioned conjugate with the pupil plane ep of the projection lens, thus constituting a so-called Koehler illumination system. When a parallel pulsed energy beam enters the optical integrator 28 with slightly varying incident angle by deflection for example with a mirror, the positions of the light spots SP formed on the exit faces of plural lens elements (for example rod lenses) constituting the integrator 28 also vary two-dimensionally. As the angle of said deflection is very small, said deflection can be obtained by driving a small mirror for example with a piezoelectric element.
FIG. 14 is a magnified view of a lens element 28' in the integrator 28. Pulses are suitably generated along the two-dimensional scanning path SL, so that the light spots SP are scattered in different positions in a raster scanning. It is to be noted that FIG. 14 is a schematic view, and the actual size of the light spot SP may become close to the size of the end face of the lens element 28'. In the example shown in FIG. 14, 11 pulses are generated in a single scanning. Said pulses may be positioned regularly or randomly on the end face of the lens element 28', during a single scanning operation.
In such method, plural pulses generated in a scanning with the mirror cause displacement of the speckle fringes (particularly interference fringes resulting from the arrangement of the lens elements 28') on the reticle serving mask or on the photosensitive wafer, and the unevenness in the illumination is reduced by the integration of plural pulses.
However, the means disclosed in the aforementioned Japanese Laid-Open Patent Sho No. 60-169136, lacking consideration on the fluctuation in the amount of energy contained in the final pulse, is still incapable of appropriate control of exposure and unable to achieve appropriate exposure. As pointed out in said patent, the fluctuation in exposure in the photolithographic process in the semiconductor device manufacture may result in a serious influence on the resolving power and in the reproducibility of the line width. On the other hand, the level of integration of integrated circuits is progressing in recent years, with increasing requirement for the resolving power. There is therefore required exposure control capable of controlling the exposure energy more precisely and achieving uniform illumination without speckle fringes.